Bonded sand cores and molds, comprising mixtures of sand, and clay or chemical binders, and optionally refractory coatings, are an important part of metal casting technology. The behavior of the bonded sand cores and molds or green sand cores and molds, when placed in contact with molten metal, is important to control the quality of metal castings formed using these sand cores and molds. Friability of the sand mixture is one factor that affects the quality of castings using a bonded sand core or mold.
Friability is the ability of a solid substance to be reduced to smaller pieces, and the friability of a sand mixture is considered a measure of the sand's abrasion resistance. A friable sand mixture is a sand mixture that is not able to withstand the erosive flow of molten metal during casting. Friable sand mixtures lose sand grains to the flowing molten stream, and the loose sand causes additional erosion and inclusion defects. As the friability of the sand mixture used in a mold increases, the ability to draw deep pockets decreases, and sand from the top half of the mold falls into the bottom half of the mold, and leads to a defect in the casting. Mold sand mixtures can become very friable if there is too high an influx of core sand or new sand and bond during the mixing process. New bond requires several passes through a mixer before its properties are developed.
Friability is inversely related to compactibility. The lower the compactibility, the higher the friability. Some mold sands, depending upon their composition and moisture and/or clay content are extremely moisture sensitive in relation to their compactibility.
In the current standard AFS friability test, two standard AFS sand specimens (specimens which are cylindrically shaped, 2 inches in diameter by 2 inches tall) are placed side-by-side in a 7 inch diameter cylindrical screen, and then the screen is rotated for one minute causing the specimens to rotate and rub against each other. The test is normally performed immediately after specimen preparation, but can be tested after various air drying intervals. As the specimens rotate, the sand abraded from the surface is collected in a pan. Weight loss is normally expressed as the weight loss of the sand specimens divided by the original starting weight (of both specimens), and multiplied by 100 to produce the “percent friability.” Work of the AFS Green Sand Test Committee suggests that a friability level of under 10% is generally satisfactory for use in molds and cores. If friability of the sand mixture is greater than 10%, a mold incorporating the sand mixture will be subject to erosion and inclusion-type defects when used with molten metal.
The presently used AFS friability test is run at room temperature, and involves the rubbing together of two room temperature sand specimens. The current friability test also does not take in to account the pressure that is created when pouring molten metal from height. The presently used friability test also does not have any mechanism or variation to represent the ratio of metal to sand. Therefore, the current test of rubbing two specimens together does not accurately depict what is happening in a real-world casting situation.